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From flat +‎ line. Refers to a continuous straight line (asystole) on an ECG (EKG) or EEG, indicating death.


  • IPA(key): /ˈflætˌlaɪn/
  • (file)


flatline (plural flatlines)

  1. An unchanging state, as indicated in a graph of a variable over time.
    • 1967 June 17, "Business Outlook", Business Week, part 6, page 23:
      Thus, you hear words like "flatline recession," "improvement at a greatly reduced rate," "economic hiccup," "recessionette," "rolling readjustment" and "the economy is double clutching."
  2. Asystole; the absence of heart contractions or brain waves.
    1. The disappearance of the rhythmic peaks displayed on a heart monitor.
    2. The disappearance of brain waves on an electroencephalogram.
      • 1972, Louis Lasagna, "Aging and the field of medicine", in Aging and Society: Aging and the professions, edited by Matilda White Riley and ‎Anne Foner, page 68:
        When brain function ceases, the electroencephalogram shows flatline recordings.


flatline (third-person singular simple present flatlines, present participle flatlining, simple past and past participle flatlined)

  1. (intransitive, of the heart) To stop beating.
  2. (intransitive, by extension) To die.
  3. (transitive, slang) To kill.
    • 2013, Shobha Nihalani, Nine
      If looks could kill, Mona's sharp gaze would have flatlined him in one second.
    • 2017, Stephanie Rowe, Not Quite Dead
      After all she'd been through, she needed sleep and food to heal, not endure a lovemaking session that was so passionate that it had pretty much flatlined him as well.
  4. To stay at the same level, without development; or, to fall.